KINE TO GENESIS. 273 



domen of the macrurous Crustacea, as the lobster, we 

 will find that the anterior face of one abdominal ring 

 is pulled into the posterior orifice of the ring lying an- 

 terior to it, forming a kind of tubular ball and socket- 

 joint, but with a flexible part of the integument with 

 no calcareous deposit, folded upon itself, and acting 

 physiologically as a tubular ligamentum teres. On ex- 

 amining the different joints, we will find that com- 

 mencing at a fixed point, as at the base of the thorax, 

 the movable ring of the first abdominal somite is pulled 

 into the fixed part. Then the first abdominal somite be- 

 comes the fixed point for the movable ring posterior to 

 it, and so on, so that we find that as the rings proceed 

 away from the thorax, each is pulled into the opening 

 of the one in advance. This is true of all those forms 

 where the abdomen is well formed, strong, and an ac- 

 tive organ in the economy of the animal ; when this 

 organ, the abdomen, ceases to be an active organ of 

 motion, as in the burrowing forms, as in Callianassa, 

 Gebia, some of the Squillidae, etc., or where it is folded 

 upon the sternum of the thoracic region, the muscles 

 becoming weaker through disuse, the rings are not 

 subject to the powerful muscular strain, and they as a 

 rule overlap but little, if at all, but lie so that the edge 

 of one ring rests upon the edge of another. In those 

 forms where degeneration of the abdomen has pro- 

 ceeded so far as not to have even the usual deposit of 

 calcareous matter, as in the hermit crabs, there are 

 simply indications of rings on the abdomen, and this 

 organ is but little more than a fleshy sac containing 

 some of the viscera, and supplied with a few muscles 

 which act together, with the form of the organ, to keep 

 the abdomen curled so that it may hold as a hook, the 



